1. Field of the Invention
The present invention relates to a semiconductor device and a manufacture method thereof.
2. Description of the Related Art
A thin film transistor (TFT) is formed using a semiconductor film.
In order to achieve high performance in a semiconductor device in which thin film transistors are used, various technologies have been examined.
For example, the following technology can be given: by thinning a channel formation region of a semiconductor layer, improvement in field-effect mobility and a field-effect subthreshold property, and reduction in leak current can be achieved.
A channel formation region, a source region, and a drain region of a thin film transistor are formed using the identical semiconductor film in many cases.
In such a case, when the channel formation region of the thin film transistor is thinned, the source and drain regions are also thinned unintentionally.
If the source and drain regions are thinned, in etching when the source and drain regions are used as a base, a margin for overetching (an etching margin) is reduced drastically. Consequently, there arises a problem that contact holes reach bottom portions of the source and drain regions in forming the contact holes through an interlayer insulating film.
In this specification, the “margin” means a permissible range of process conditions in a production process. If a process is carried out under a process condition beyond the permissible range in the production process, a defect occurs in the semiconductor device. Therefore, as the margin increases, the incidence of defects decreases, so that restrictions on the process conditions decrease. On the other hand, as the margin decreases, the incidence of defects increases, so that restrictions on the process conditions increase.
When a contact hole reaches a bottom portion of a source region (or a bottom portion of a drain region), it is possible to contact a side of the contact hole with a contact electrode. However, an area of a side of the thinned source or drain region is close to zero. Therefore, contact resistance of the source region (or the drain region) and the contact electrode cannot sufficiently be lowered, so that a sufficient voltage cannot be applied to the source region (or the drain region). As a result, the thin film transistor cannot operate.
In order to solve the above problem, Reference 1 (Japanese Published Patent Application No. H5-13762) discloses a method in which a contact hole is formed by plural times of etching under different etching conditions.